Orthodox Apologetics

Saint Peter became a rock of faith in Matthew 16:18... and a stone of stumbling in Matthew 16:23. What changed? The words coming out of his mouth. In the former, he confessed Jesus to be the Christ, the Son of the living God... whereas in the latter, he committed a blunder, albeit a well-meaning one. This shows us that the Pope's quality of being a true rock of faith is conditioned, and not unconditioned, as official Roman Catholic teaching upholds.

The dogma of Papal Infallibility bears thus an uncanny resemblance to the Calvinist doctrines of Unconditional Election and Perseverance of the Saints : only that in this case, it's Unconditional Infallibility and Perseverance of the Pope.